Hi, I wouldn’t normally answer this sort of question as you could look up how to do this in your help files in about two minutes, but since I’m bored at the moment…
Instructions for Ill 10 (probably the same for at least 8 and 9)
Create the circle with the elipse tool (L). Fill and stroke do not matter as they will disappear after you create your text. Click and hold on the type tool button on your Tools Palette until the flyout menu appears. The third and sixth tools in that flyout are for text on a path (regular and vertical text respectively). Choose either of those. You will now see you text cursor has a little dotted line running through it. Go over to your circle and carefully click on the outline (stroke) of the circle. (Illustrator will let you know with an error message if you miss). You can now type your text and edit it as per normal text. If you want to adjust the position of the text then choose your Direct Selection tool and click on the text and an ‘I-bar’ will appear at the start of the text. Still using the selection tool you can drag that I-bar around the perimeter of the circle to adjust the starting position of the text. Also you can drag the I-bar across the perimeter of the circle to invert the text and place it on the inside of the circle. In either position you can use the ‘Baseline Shift’ options in the character palette to adjust the distance of the text from the outline of the circle.
All these instructions will work for text on any sort of path i.e. ellipse, rectangle, arc, freehand pencil line etc etc